This invention relates to the process of electrochemical machining and, more particularly, concerns a method of monitoring an electrochemical machining process and a tool assembly therefor.
Electrochemical machining (ECM) is a non-mechanical process in which the tool never comes in contact with the workpiece during the machining process. The tool as a cathode and the workpiece as an anode are connected to an electrical power source. A gap that exists between the tool and the workpiece is filled with a pressurized, flowing, aqueous electrolyte. ECM is generally the reverse of electroplating. The flowing electrolyte, acting as an electrical current carrier, removes metal ions from the anodic workpiece and carries them away via the gap. The gap ranges in size from 0.1 millimeters to several millimeters. The tool is typically made of brass, bronze or stainless steel. The electrolyte is a highly conductive inorganic salt solution, such as sodium nitrate. A cavity which is produced in the anodic workpiece is a female mating image of the cathodic tool.
Given a tooling geometry, dimensional accuracy of the workpiece is primarily determined by the gap distribution. The gap size should be maintained at a proper range. Too small a gap, such as less than 100 micrometers in a standard ECM operation, would lead to arcing or short-circuiting between the tool and the workpiece. Too large a gap would lead to excessive gap variation as well as reduction in the machining rate. Monitoring and controlling the gap size between the tool and the workpiece, or directly monitoring the workpiece thickness, is important for ECM tolerance control. For example, in machining a turbine compressor blade, the blade thickness should be directly measured during machining so that a desired thickness can be obtained.
Lack of suitable means for sensing gap size or workpiece thickness may hinder ECM accuracy control. Without such means, many rounds of costly trial-and-error experiments must be run to obtain the gap size changes that occur during the machining process. Gap size can change significantly during the machining process, partly because conductivity of the electrolyte may change in the gap due to Joule heating or gas bubble generation on the tool surface. Variation and inaccuracy in tool feed rate and tool positioning can also contribute to changes in gap size and workpiece thickness. In-process gap detection or workpiece thickness detection is thus important for improving ECM process control.
Several types of ECM sensors have been developed over the years since ECM came to industrial uses four decades ago. An eddy current ECM gap sensor was reported in Annuals of the CIRP (1982, Vol. 37/1, pp.115-118, by C. Bignon). An ECM control method using an ultrasound sensor is described in U.S. Pat. No. 5,672,263 to Raulerson et al. and is used for ECM of a large casing. However, the Raulerson et al. method is limited to applications which have a large space for housing the sensor and storing the fluid through which the ultrasonic wave propagates. By way of example, the Raulerson et al. method cannot be applied to the ECM of turbine compressor airfoils because space is limited in the machining area and also because the airfoil is surrounded by cathodes that make it impossible to directly measure airfoil thickness. The Raulerson et al. method also does not measure the gap size and is intended only to measure the workpiece thickness that is near a wide open space. Consequently, need remains for a method of monitoring an ECM process which overcomes the aforementioned limitations of the prior art without introducing any new problems.
Monitoring an electrochemical machining process and a tool assembly therefor is achieved by embedding an ultrasonic sensor in the ECM tooling assembly. Measurement of both the gap size and workpiece thickness is performed using ultrasonic signals and is not limited by the amount of space in the machining area and is particularly applicable to the ECM of turbine compressor airfoils.
In a preferred embodiment of the invention, a method of monitoring an electrochemical machining process comprises the steps of: embedding an ultrasonic sensor in an electrochemical machining tool to provide a tool assembly; providing the tool assembly in a spatial relationship with a workpiece; flowing an electrolytic fluid at least between the tool and the workpiece; connecting the tool and the workpiece to an electrical power source; generating an acoustic wave from the ultrasonic sensor so as to propagate from the tool through the electrolytic fluid to the workpiece; receiving reflections of the acoustic wave from the workpiece; and, based on the propagated acoustic wave and the reflections thereof, calculating measurement of at least one of (a) the size of a gap between a cutting surface of the tool and a first working surface of the workpiece facing the cutting surface of the tool and (b) the thickness of the workpiece between the first working surface of the workpiece and a second working surface thereof facing away from the first working surface. The method also comprises the step of applying an acoustic couplant between the ultrasonic sensor and the tool.
More particularly, the receiving step includes reflecting a first part of the acoustic wave at the cutting surface of the tool and returning it to the ultrasonic sensor at a first arrival time, and reflecting a second part of the acoustic wave at the first working surface of the workpiece and returning it to the ultrasonic sensor at a second arrival time. The calculating step includes subtracting the first arrival time from the second arrival time, multiplying the difference by the velocity of the acoustic wave in the electrolytic fluid, and dividing the product by a factor of 2 to obtain the gap size between the cutting surface of the tool and the first working surface of the workpiece.
The receiving step also includes reflecting a first part of the acoustic wave at the first working surface of the workpiece and returning it to the ultrasonic sensor at a third arrival time, and reflecting a second part of the acoustic wave at the second working surface of the workpiece and returning it to the ultrasonic sensor at a fourth arrival time. The calculating step includes subtracting the third arrival time from the fourth arrival time, multiplying the difference by the velocity of the acoustic wave in the electrolytic fluid, and dividing the product by a factor of 2 to obtain the thickness of the workpiece between the first and second working surfaces of the workpiece.
In another exemplary embodiment of the invention, an electrochemical machining tool assembly is provided which comprises: an electrochemical machining tool positionable in a spatial relationship with respect to a workpiece and positionable in contact with an electrolytic fluid disposed at least in a gap between the tool and the workpiece, the tool having a cutting surface facing the workpiece; and an ultrasonic sensor embedded in the tool for generating an acoustic wave that propagates from the tool through the electrolytic fluid to the workpiece and is reflected back to the ultrasonic sensor for use in calculating a measurement of at least one of (a) the size of the gap between the cutting surface of the tool and a first working surface of the workpiece facing the cutting surface of the tool and (b) the thickness of the workpiece between the first working surface of the workpiece and a second working surface of the workpiece facing away from the first working surface of the workpiece. The assembly also comprises an acoustic couplant applied between the ultrasonic sensor and the tool.